Blind fasteners which are installed by operations conducted at only one side of the work are in widespread usage. In some types, a tubular sleeve is pulled over a tapered nose piece to enlarge its diameter and thereby form a retaining head. An example of this type of fastener is shown in Wing and Schuster U.S. Pat. No. 3,129,630.
This is a successful fastener, but has the disadvantage that the bearing area of the head is limited to the area of the end of the expanded sleeve. This results in relatively high unit loads on the workpiece which it abuts. This is fully satisfactory for most installations in metallic workpieces. However, when the workpiece is a composite material, the unit loading should be minimized, because composite materials are not optimally designed for this type of load. It would be preferable for the expanded head to have a larger "footprint".
In order to provide a larger footprint, a buckling type blind fastener was developed, which is exemplified by Wing U.S. Pat. No. 3,236,143. In this device, a circular tubular sleeve is put into compression between the workpiece and the head of a pull stem, and the tube is caused to fail in a buckling mode. This produces one or more tubular enlargements, one of which bears against the workpiece to retain the fastener. The problem inherent in this type of fastener is that much of the force required to buckle the sleeve is also applied to the workpiece, and this can also be a disadvantage when the fastener is to be set in composite materials, or in any material where applied localized compressive forces should be minimized.
Accordingly it is an object of this invention to provide a buckling sleeve blind fastener whose buckled head is formed principally by reactions with its own nut, and which is largely pre-formed before it reaches the workpiece. Thus, the forces required to form the buckled head are mostly isolated from the workpiece. Of course the pull-up forces needed to tension the fastener and clamp the work together will be applied to the workpiece, but through the type of enlarged head that can be provided by a buckled sleeve.
This invention will be fully understood from the following detailed description and the accompanying drawings, in which: